1. Field of the Invention
The present invention relates to a power amplifier system and in particular but not exclusively to a power amplifier system for use in a communications system such as a wireless telecommunication system.
2. Description of the Related Art
An area covered by a cellular telecommunications network is divided into a plurality of cells. Each of these cells has a base station arranged to transfer signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in general in communication with a base station associated with the cell in which the mobile station is located.
A modulation system has been developed for use within a cellular network using the global system for mobile communication (GSM) standard which enhances the rate at which data is transferred between the mobile stations and base stations. This modulation scheme is called enhanced data rate for GSM evolution or EDGE modulation. EDGE modulation is a known modulation scheme so will not be explained in detail hereinafter. It is sufficient to note that EDGE modulation modulates digital data using both phase and amplitude information. To reduce errors at the receiver, the transmitted signal is similar to the modulated signal. Therefore the phase and amplitude errors introduced between the edge modulator and the transmitting antenna should be kept to a minimum.
Work is currently been carried out on a third generation cellular telecommunications standard. It is proposed that the third generation standard use code division multiple access in the radio domain between the mobile station and the base station. In one proposal, wide band CDMA is being proposed. Again, CDMA is a relatively complex modulation scheme, which allows higher data rates to be achieved.
However, these modulation schemes are relatively complex. In order to provide the higher data rate, this puts higher constraints on the radio frequency circuitry and in particular the power amplifier. In particular, there are two issues which need to be addressed: firstly, the peak powers of fluctuating envelope and secondly, linearity. The fluctuating envelope has two main effects on the power amplifier performance. The average power, that which defines the power dissipated in the power amplifier, is forced to run back from the maximum power of the power amplifier (this is referred to as power amplifier back off). This is disadvantageous because the maximum power is the most efficient operating point for the power amplifier. The reason why the average power has to run back from the maximum power is because of the peak to average ratio of the modulated signal. In more detail, there will generally be a linear region for the power amplifier where there is a linear relationship between the input signal power and the output signal power. This is for lower input signal powers. Generally, the power amplifier should be operated in the linear region in order to reduce intermodulation distortion. However, as the input signal power increases, the amplifier starts operating non-linearly beyond the linear region. This point is generally the point where the power amplifier is most efficient.
The fluctuating envelope signal is going to exacerbate the non-linearity of the power amplifier producing large intermodulation products. This results in the power amplifier having to be backed off even further than required by the first point mentioned above. This makes the power amplifier even less efficient. In fact, this can result in the power amplifier, in some situations, never being capable of meeting linearity requirements on its own. Hence, power amplifiers have linearisation schemes bolted onto them so that only the average power determines how much the power amplifier needs to be backed off. There are many different examples of linearisation schemes. One example is digital predistortion where the input signal to the power amplifier is distorted digital at base band so that once it passes through the power amplifier, following up conversion, any intermodulation products produced by the power amplifier are cancelled.
However, power amplifier characteristics become more complicated when predistortion linearisation is used. One problem is the so-called memory effect. This occurs where intermodulation products generated by the power amplifier are not symmetrical about the carriers, in both amplitude and phase. Power amplifiers with significant memory effects require the predistortion signal applied to the input must also contain the same memory characteristics. In other words, the signals which are provided to the power amplifier to cancel the inter-modulation products are not symmetrical in terms of amplitude and phase about the carriers. This means that the circuitry for generating the predistortion linearisation can be complicated. This can also limit the extent to which the system will cancel the unwanted intermodulation products.
It should be appreciated that this problem is exacerbated when dealing with high bandwidth systems. Indeed, the digital predistortion systems are generally limited in the bandwidth of the predistortion signal that they can produce.